This invention relates generally to focused ion beam systems and relates, in particular, to a scanning focused ion beam system which incorporates a sputter induced photoemission detector.
Focused scanning ion beam systems are known in the art, and have been implemented in research and production settings for several years. Conventional applications of focused ion beam systems include ion beam lithography, ion beam microscopy, ion beam milling and deposition, and secondary ion mass spectroscopy (SIMS).
Prior Art SIMS systems are typically utilized for surface analysis of semiconductors and other substrates. SIMS devices include a central vacuum chamber which houses a secondary ion mass spectrometer and a source of ions. The ions can be electrostatically steered so as to impinge upon a substrate such as a semiconductor device or a biological target. When the ions strike the target, secondary ions are created from the surface atoms due to nuclear collision events. These ions are collected by the SIMS detection system and are analyzed to determine their mass, and thus their elemental composition.
In a number of conventional focused ion beam systems, the primary ion beam can be deflected to strike selected points on the sample for analysis of those points. Alternatively, the ion beam can be scanned in a raster fashion to create a mapping of the area being scanned. It is well known in the art that in order to obtain accurate results from a mass analyzer system utilizing charged particle optics, the target must remain at a fixed voltage. When the ion beam target is a semiconductor or an insulator, it can accumulate charge from the high energy primary ion beam and its electrical potential will become more positive. In particular, if excess positive charge accumulates on the target, the ion beam can be deflected from the intended trajectory or even fail to reach the target.
A number of techniques are known in the art to reduce this problem of charge accumulation on the ion beam target in SIMS and other focused ion beam systems. One such technique is electron beam charge neutralization. In electron beam charge neutralization, an electron beam impinges on the target at the same general spot as the primary ion beam. The electron current, energy and current density are adjusted to offset the charge induced by the ion beam until the surface voltage is fixed and stable. Unfortunately, this technique is sometimes unsuited for elemental analysis in production oriented ion beam devices, because frequent adjustments must be made, often at each new target site.
There accordingly exists a need for focused ion beam target analysis and mapping systems which yield accurate elemental identification regardless of positive charge accumulation on the target.
SIMS systems are also used for depth profiling, a technique in which a map of elements is created at various depths in the target. In this process, the primary ion beam is used to stimulate the surface ions for the SIMS detector and to ion mill, or sputter, the target. The ion beam in these depth profiling devices removes the surface layers of the target material in a controlled fashion. Then, a new SIMS map is created of the new surfaces at selected depths, until a cross-section of the area, or depth versus element profile, is generated.
Another important application of the SIMS technique, essentially a variant of depth profiling, utilizes a SIMS detector as an end of process, or endpoint, detection system. In this application, the SIMS detector is used to continuously monitor the surface of a sample composed of layers of different elements, such as aluminum and silicon, while the sample is being milled by the primary ion beam or other material removing device. The material removal operation continues, removing the topmost level and exposing lower levels to the SIMS detector, until the desired element or end point is exposed.
Conventional SIMS-based end of process detectors suffer from some significant deficiencies. One limitation is imposed by the extremely low secondary ion yield of certain elements. Such low yields render conventional SIMS-based endpoint detectors unsuitable for a number of important applications.
There accordingly exists a need for a system which can provide accurate and reliable end point detection even for target materials having low secondary ion yields.
It is thus an object of the invention to provide an improved focused ion beam system.
It is a further object of the invention to provide such a focused ion beam and detector system which can generate high resolution observations of spatial information.
It is another object of the invention to provide a focused ion beam and detector system for generating scanning ion images and mappings for a wide range of target materials.
It is still another object of the invention to provide a depth profiling system, for use with ion beam milling apparatus, which can provide accurate and reliable end point detection and process control in a variety of materials.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.